Epoxy resin compositions which use polythiol as a curing agent and a liquid tertiary amine as an accelerator are known as low-temperature, rapid-curing epoxy resin compositions which may be cured at -20.degree. C. to 0.degree. C., and these are widely used in adhesives, sealing agents, casting materials and the like.
However, such epoxy resin compositions are disadvantageous in that their pot life is very short, usually a few seconds to a few minutes after mixing, and thus there is not enough time for mixing, defoaming and application. Also, since the user must prepare a new composition each time, the working efficiency is lowered, and since the excess composition cannot be preserved it must be disposed of, which is disadvantageous from the point of view of conservation of resources and environmental problems.
Thus, it has been desired to develop a polythiol epoxy resin composition which has a sufficiently long working life and a good working efficiency. Nevertheless, the commercially available epoxy resins which contain thiol compounds generally have a poor shelf life, and it is difficult to increase the working life of epoxy resin compositions when used with thiol curing agents.
As a method to solve such problems, a method has been investigated in which an acid anhydride or mercapto organic acid is added to the resin as a retarder to lengthen the working life thereof (Japanese laid-open patent application S61-159417), but this method cannot be said to be sufficiently satisfactory.
On the other hand, examples of a thiol compound obtained by an esterification reaction between a polythiol and a mercapto organic acid which is used as a curing agent for epoxy resins are described in Japanese laid-open patent application S41-7236, Japanese laid-open patent application S42-26535, Japanese laid-open patent application S47-32319, Japanese laid-open patent application S46-732 and Japanese laid-open patent application S60-21648, but the accelerators used in these epoxy resin compositions are liquid amines, etc. Also, the compositions using these liquid amines, etc. have a very short working life of from a few minutes to a few dozen minutes, which is a serious disadvantage.
Thus, there remains a need for polythiol epoxy resin compositions which exhibit good curability at relatively low temperatures and also exhibit a long working life.